It is standard practice to level, that is make planar and straight, thin metal strip during rolling, straightening, and/or stretch leveling. This is typically done by gripping the strip as it moves in a normally horizontal transport direction between an upstream roll pair and a downstream roll pair that are both driven to exert tension on the strip. The instant invention is aimed at thin metal strip of a thickness ranging from 0.05 mm to 1 mm, preferably, 0.1 mm to 0.5 mm, and in particular to such strip made of aluminum alloy.
Based on current standards, metal strip, and in particular thin metal strip, must meet ever higher requirements with respect to strip planarity along with the highest quality for strip surface. A variety of methods are known in this connection for leveling metal strip-rolling (in particular, skin-pass rolling), straightening (in particular, tension flex leveling), and stretch leveling. The apparatuses used for this purpose frequently have a feed roll driven or braked for establishing tension and a feed roll that releases tension. This is true, in particular, for tension flex leveling and stretch leveling, as well as for skin-pass rolling in the case of in-line skin-pass rolling methods. With skin-pass rolling, the skin-pass mill stand is then between these roll sets, while in the case of tension flex leveling the tension-flex-leveling unit can be provided between these roll sets. With a stretch-leveling apparatus, usually at least one additional roll set in the form of a stretch-leveling roll set is provided between the feed roll set and the feed roll set.
With the known methods for leveling metal strip by rolling, straightening, and/or stretch leveling, it is almost impossible in particular to completely eliminate waviness (edge waves and center waves) or strip saber—planarity profiles unsymmetrical relative to the strip center—with the result that an ideal strip planarity is only rarely achieved. Another known approach for improving strip planarity, for eliminating waviness and strip saber, e.g. during skin-pass rolling, is to generate a changeable temperature profile over the width of the strip for the purpose of controlling the tensile stress distribution, thereby enabling the degree of leveling to be adjusted by modifying the tensile stress distribution (see U.S. Pat. No. 6,327,883).
In addition, an approach has been proposed for reducing edge waves and center dishing during the leveling of metal strip, where an adjustable contour having, e.g. a convex outer camber and/or concave inner camber is provided in the roll set (see U.S. Pat. No. 5,341,166).
Also known is an apparatus for the tension flex leveling of metal strip using guide rolls mounted parallel to each other and a straightening roll bearing against of two guide rolls, where the strip wraps in a positive-fitting manner around the straightening roll between two contact lines, along which lines the guide rolls are in indirect contact through the strip with the guide roll. In order to be able to modify the insertion depth, and thus also the wrap angle determined by the guide-roll radii or the contact lines, as a function of the strip thickness and the strength of the strip material, the backing rolls, and guide roll, and the straightening roll are supported on a shared console that can be pivoted about a pivot point (see U.S. Pat. No. 5,953,946). As a result of these measures, the tensile stress distribution is not varied over the width of the strip, with the result that the degree of leveling also cannot be varied over the width of the strip.